Have you checked around the soap dish. That is the usual point for water to get behind the tile.

: I just recaulked my tub and the stuff won't cure. Apparently I've got water behind the tile but can't determine the source. I've looked at the supply pipes with both the faucet and shower running and they're dry as a bone. If those aren't leaking, how can water get behind my tile? Do you think it seeped through the old caulk and I just need to let it dry out and caulk again? Or do I have other problems?